U.S. application Ser. No. 451,283, filed concurrently to Ser. No. 451,284 by me under the title Transparent Colorless Glass-Ceramics Especially Suitable for Use as Stove Windows, discloses a relatively narrow range of precursor glass compositions which, although containing TiO.sub.2 and up to 0.1% by weight Fe.sub.2 O.sub.3, can surprisingly be crystallized in situ through heat treatment to yield essentially colorless glass-ceramic articles wherein .beta.-quartz solid solution constitutes the predominant crystal phase. Thus, as is discussed therein, in crystallizing the typical parent glass article containing TiO.sub.2 as a nucleating agent and Fe.sub.2 O.sub.3 as an impurity in amounts greater than 0.01%, a reaction is believed to take place between the titanium and iron ions during the heat treatment process, whereby the valence state of the titanium ions is reduced from +4 to +3. The result of that reaction is the formation of an unattractive gray-brown coloration in the glass-ceramic. The crux of that inventive subject matter lies in the discovery that the essential absence of MgO from the compositions permits the production of virtually colorless, transparent glass-ceramic bodies from precursor glasses containing up to 0.1% Fe.sub.2 O.sub.3, wherein TiO.sub.2 comprised at least a substantial portion of the nucleating agent. And, because .beta.-quartz or .beta.-eucryptite solid solution crystals, as such have been variously denominated, can exhibit coefficients of thermal expansion over the range of 0.degree.-300.degree. C. of 0 or less, those glass-ceramics can demonstrate coefficients of thermal expansion over the same temperature interval within the regime of about -10 to +10.times.10.sup.-7 /.degree.C. Those glass-ceramic articles are produced by heat treating the following glass compositions, expressed in terms of weight percent on the oxide basis, at temperatures between about 750.degree.-950.degree. C.:
______________________________________ SiO.sub.2 65-75 Li.sub.2 O 1-4 Al.sub.2 O.sub.3 15-25 ZnO 0.5-2 Na.sub.2 O and/or K.sub.2 O 0-2 TiO.sub.2 2-6 ZrO.sub.2 0-2 BaO 0-2 F 0-1.2 Fe.sub.2 O.sub.3 &gt;0.01 but &lt;0.1 ______________________________________
The most preferred composition was stated to have an analyzed composition of about:
______________________________________ SiO.sub.2 66.6% Al.sub.2 O.sub.3 20.5 Li.sub.2 O 3.7 Na.sub.2 O 0.4 K.sub.2 O 0.5 ZnO 1.0 BaO 2.0 TiO.sub.2 2.6 ZrO.sub.2 1.6 As.sub.2 O.sub.3 0.5 Fe.sub.2 O.sub.3 0.05 ______________________________________
Those glass-ceramics are described as being especially useful as windows for coal and wood burning stoves since their transparency and lack of color provided an uninterrupted and undistorted view of the fire inside the stove. However, because the precursor glass therefor possesses the thermal stability and viscosity characteristics necessary for forming sheet, coupled with their high refractoriness, excellent chemical durability, very low coefficient of thermal expansion (about .+-.7.times.10.sup.-7 /.degree.C. over the range of 25.degree.-700.degree. C.), and capability of being crystallized in situ very rapidly, other applications for those glass-ceramics in sheet form such as countertops and range tops have become apparent.
As can be appreciated, not all such applications require a colorless transparent product. Indeed, where a decorative effect is wanted, colors immediately come to mind. (Even in windows for coal and wood burning stoves, there has been some desire voiced for a product having a light brown tint to impart a warm, cheerful appearance to the fire inside.) Furthermore, many applications do not require transparent products. Accordingly, it would be highly desirable to be able to produce transparent and opaque glass-ceramic articles having compositions approximating the analyzed values above, but exhibiting a range of colors.